FIG. 5 shows a cross-sectional view of an exemplary conventional vacuum fluorescent display device taken along the line V—V shown in FIG. 6. FIG. 6 illustrates a cross-sectional view taken along the line VI—VI depicted in FIG. 5. In FIG. 5, control electrodes 9 and anodes 8 illustrated in FIG. 6 are omitted for simplicity. The vacuum fluorescent display device illustrated in FIGS. 5 and 6 includes a sealed box-shaped envelop 1 made of an insulating material, e.g., glass. The inner space of the envelop 1 is maintained in a high vacuum state. The envelop 1 is provided with an insulating anode substrate 2 and a front substrate 3, which face each other at a certain distance therebetween. A frame-shaped side plate 4 is installed along peripheries of the anode and the front substrate 2 and 3. The side plate 4, the anode substrate 2 and the front substrate 3 are air-tightly coupled together by using a sealant 5. Formed on an inner surface of the anode substrate 2 in a certain pattern are anodes 8 each being composed of an anode conductor 6 and a fluorescent substance layer 7 disposed thereon. Further, control electrodes 9 are installed above the anodes 8 and filament-shaped cathodes 10 are extended above the control electrodes 9.
Each filament-shaped cathode 10 has a structure in which a thermionic emission layer composed of a composite oxide of, e.g., alkaline earth metals (Ca, Sr, Ba, and the like) is disposed around a core wire made of tungsten or tungsten-based alloy.
With the application of certain filament voltage to the filament-shaped cathode 10, the thermionic emission layer is heated to a temperature of about 600–650° C. Electrons emitted from the thermionic emission layer of the cathode 10 are controlled by the control electrodes 9 and collide against the anodes 8 to allow the fluorescent substance layer 7 to radiate light. The emitted light is seen at the outside of the envelop 1 through the light transmitting front substrate 3.
In order to prevent the filament-shaped cathode 10 from being loose due to thermal expansion caused by electric heating, the filament-shaped cathode 10 needs to be supported by cathode supports employing a spring-like member having adequate tension force and stroke. To this end, the cathode supports include an anchor and a support.
In the conventional vacuum fluorescent display device as shown in FIGS. 5 and 6, the cathodes 10 in the envelop 1 are supported under tension by a pair of cathode supports 11 and 12. The cathode supports 11 and 12 are made by press-working metal sheets, one of which being an anchor 11 for holding one ends of filament-shaped cathodes 10 and the other being a support member 12 for supporting the other ends thereof. The anchor 11 is provided with a base 11a fixed on the anode substrate 2; spring-shaped arms 11b seamlessly formed with the base 11a; and tabs 11c, provided at distal ends of the arms 11b, for supporting one ends of the cathodes 10, respectively. The support 12 is provided with a base 12a fixed on the anode substrate 2; and a tab 12b, seamlessly formed with the base 12a, for supporting the other ends of the cathodes 10.
In the vacuum fluorescent display device shown in FIG. 5, a plurality of (four in FIG. 5) cathodes 10 are arranged in parallel and supported by the anchor 11 and the support 12. The cathodes 10 are regularly spaced apart from each other in a direction perpendicular to a lengthwise direction of the anchor 11 and the support 12. Accordingly, the arms 11b of the anchor 11 need to be arranged in an approximately same direction, which is slanted with respect to the lengthwise direction of the cathodes 10, and bent so that they can function as resilient members, e.g., springs, to apply tension to the cathodes 10. As a result, as viewed from the top, one of two sidemost arms 11b among those arranged in parallel is outwardly protruded from an area where the cathodes 10 are extended. Consequently, there occurs a dead space D on the anode substrate 2 in the envelop 1, which can not be used as a display area A due to the absence of cathode 10 provided thereabove. Accordingly, the use of the conventional cathode supports 11 and 12 inevitably limits the size of the display area A to become considerably smaller than that of the envelop 1. Moreover, it is difficult to enlarge the display area A.
In order to solve such a problem, the inventor of the present invention has developed a cathode support 13 shown in FIGS. 7 and 8. FIG. 7 is a plan view of the cathode support 13 made of a metal sheet and installed on the anode substrate 2, for holding four cathodes 10, and FIG. 8 is a development view thereof. The cathode support 13 includes a base 13a fixed on the anode substrate 2; two anchors 14 spaced apart by a predetermined distance therebetween, each anchor 14 having an arm 14a which is provided with a distal end at which a tab 14b for attaching one end of a cathode 10 thereon is disposed and a proximal end portion of which one side is seamlessly connected to the base 13a; a first support 15a seamlessly provided at the other side of the proximal end portion of an arm 14a of the inner anchor 14; and a second support 15b spaced apart from the first support 15a by a certain distance and seamlessly formed with the base 13a. The above-described cathode support 13 shown in FIG. 7 is used for supporting one ends of the cathodes 10, while the other ends thereof are supported by another cathode support disposed on the anode substrate 2 in a rotationally symmetrical relationship with the cathode support 13.
Using the cathode support 13 illustrated in FIG. 7 can lead to a vacuum fluorescent display device having the same number of cathodes and an equal-sized display area as in the vacuum fluorescent display device illustrated in FIG. 5. In such a case, however, no dead space is generated by the arms 14a of the cathode support 13, and therefore, a size of an envelop can be made smaller than that of the envelop shown in FIG. 5. On the other hand, in case of maintaining the size of an envelop while employing a cathode support of the type shown in FIG. 7, more cathodes 10 can be arranged than in the vacuum fluorescent display device shown in FIG. 5, thereby allowing the display area to be enlarged.
However, with the configuration of the cathode support 13 illustrated in FIG. 7, it is difficult to save power and/or increase brightness by reducing the distance between the cathodes 10 and the anodes 8 and then applying a low voltage to the cathodes 10.
Specifically, electrons emitted from the cathodes 10 are diffused as shown in FIG. 9 and then collide against the anodes 8 of the anode substrate 2. Thus, if the distance FH between the cathodes 10 and the anodes 8 is reduced while maintaining the distance FW between the cathodes 10, there occur on the anode substrate 2 some areas which electrons cannot reach. With reference to FIG. 9, if the distance between the cathodes 10 and the anodes 8 is reduced from FH1 to FH2, i.e., the position of the anodes 8 is lifted up to the line L shown in FIG. 9, the electrons do not reach area B on the anode substrate 2.
In this case, when the distance FW between the cathodes 10 is reduced, there occurs no area B where the electrons do not reach even though the distance FH between the cathodes 10 and the anodes 8 is reduced. Accordingly, it is possible to save power and/or improve brightness by applying a low voltage to the control electrodes 9 and the anodes 8.
However, with the structure of the cathode support 13 illustrated in FIG. 7, it is difficult to reduce the distance FW between the cathodes 10. In order to reduce the distance FW, the length of the arms 14a and the distance therebetween should be shortened and the location of the first support 15a directly provided as shown in FIG. 8 at the proximal end portion of an arm 14b also needs to be changed. In that case, since the arms 14a can not effectively function as springs, sufficient tension force can not be applied to the cathodes 10. Therefore, it is difficult to reduce the distance FW between the cathodes 10 down to a certain value, e.g., less than or equal to 3 mm, while maintaining the resilience of the arm 14a. Accordingly, with the use of the cathode support 13 of FIG. 7 having the support 15a directly connected with the bottom portion of an arm 14a, the distance FH between the cathodes 10 and the anodes 8 can not be reduced and a low voltage can not be applied to the cathode 10 for power saving and/or brightness improvement.
In addition, since the support 15a is provided at the bottom portion of the arm 14a of the inner anchor 14, vibrations from the inner anchor 14 are directly transmitted to the support 15a, resulting in vibration or disconnection of the cathode 10 fixed to the support 15a. 